如图，已知▱ABCD的对角线BD=4cm，将▱ABCD绕其对称中心O旋转180°，则点D所转过的路径长为（　　）

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    Charles Dickens was an English writer. He was born in 1812. He created some of the world's best-known characters and many readers think he is the most important writer of the Victorian era (1837-1901). Some of his famous novels are Oliver Twist, David Copperfield and Great Expectations. Most people in his novels lived in London.

    In 1824, Dickens' father went to prison(监狱) because he couldn't pay money to someone. In those days, a man's wife and young children went to prison with him. Charles was 12, so he left school to work in the prison. Life was hard, and a lot of children worked in those days. Dickens' own experience helped him to write about poor children without a mother or father. Dickens' success began with the publication (出版) of The Pickwick Papers in 1836. And his book Oliver Twist is about a boy without father. Important people were more interested in the problems of poor people after reading Dickens' books.

    Dickens' novels are often very long and he wrote them in different parts for a newspaper each week. His novels have a lot of exciting and sad things in them. He wanted his readers to buy the newspaper every week. And the readers wanted to finish the story! Dickens' novels are popular because many of his stories are about social(社会的) problems and family life. Dickens was successful in writing and he has been praised by other writers for his style and characters.

You should _______ yourself.

用“不饱和”或“饱和”填空．

在20℃时，在盛10g水的烧杯中加入3g某物质，充分溶解后还有1g剩余，则所得溶液是该物质在20℃时的____ 溶液；若温度不变，继续向烧杯里加入10g水，充分溶解后所得溶液是该物质的____ 溶液．

已知曲线C的极坐标方程是ρ=4cosθ．以极点为平面直角坐标系的原点，极轴为x轴的正半轴，建立平面直角坐标系，直l的参数方程是$\left\{\begin{array}{c}x=1+t\cos \alpha \\ y=t\cos \alpha \end{array}\right.$（t是参数）

（1）将曲线C的极坐标方程化为直角坐标方程；

（2）若直线l与曲线C相交于A、B两点，且|AB|=$\sqrt{14}$，求直线的倾斜角α的值．